Filaments with good characteristics can be fabricated from high polymers only when an oriented structure is generated in the strand (see the Ullmann Encyclopedia, 5th edition, volume A-10, page 456). It is desirable and indeed necessary for this purpose to align micro-oriented regions such as fibrides in the polymer along the fiber axis. This alignment of orientation can be effected by the various fabricating techniques used to produce such filaments and can depend upon the process to which the fiber or filament is subjected. In most cases the orientation is effected by a stretching.
The process steps and the conditions in which and under which this stretching is carried out has an impact upon the fiber properties which are produced. In melt spinning the fibers are stretched in a hot plastic state while the molecules are still mobile. Soluble polymers can be wet spun or dry spun. In dry spinning the stretching is effected while the solvent is removed or evaporated. Extruded fibers which are coagulated in a precipitating or coagulated bath are commonly stretched during the coagulation.
Processes of these types are well known and widely described. In all of these cases, however, it is important that the transition from the liquid state, independently of whether this is a melt state or solution state, to the solid state be so effected that during the filament formation an orientation of the polymer chain or of the polymer chain packets (with reference to fibrides, fibrils or the like) is brought about.
To inhibit the sudden evaporation of a solvent from a filament during dry spinning, there are a number of possibilities. However, the problem of very rapid coagulation of polymers during wet spinning as is the case with the spinning of cellulosic amine oxide solutions has been solved heretofore only by a combination of wet spinning and dry spinning.
It is, therefore, known to pass solutions of polymers into the coagulating medium via an air gap.
In EP-A-295,672, the production of aramide fibers is described. These fibers are brought via an air gap into a non-coagulating medium, stretched and then subjected to coagulation. East German Patent 218,124 describes a spinning of cellulose in amine oxide solution via an air gap in which precautions must be taken to prevent mutual adhesion of the elongated elements thus produced.
According to U.S. Pat. No. 4,501,886 cellulose tri-acetate can be spun using an air gap.
U.S. Pat. No. 3,414,645 describes the production of aromatic polymide articles from solution in a dry/wet spinning process. In all of these processes an orientation is effected in the air gap if only because the downwardly emergent solution strand from the orifice is at least stretched by the gravitational force on the strand of the solution emerging from the nozzle. The orientation effected by gravitational action can be increased when the velocity of the extruded solution emerging from the orifice and the withdrawal speed of the fibers passing through the coagulating bath are so adjusted that further stretching occurs.
A process of this latter type is described in Austrian Patent 387,792 and the equivalent U.S. Pat. No. 4,246,221 and U.S. Pat. No. 4,416,698.
In this system a solution of cellulose in NMKO (N-methylmorpholine-N-oxide) and water is formed. The stretching is effected with a stretching ratio of at least 3:1. For these purposes an air gap height as measured from the bottom of the nozzle to the top of the NMMO/water bath of 5 to 70 cm is necessary.
A drawback of this practice is that extremely high withdrawal speeds are required to carry off the strand and in order to insure that a minimum strand stretching ratio is obtained to provide corresponding textile characteristics of the spun filament. It has also been found that longer air gaps tend toward more sticking together of the fibers and especially at high draw ratio lead to unreliable results in the spinning operation and filament breakage.
As a consequence, precautions have been necessary to avoid these drawbacks. Austrian Patent 365663 and the equivalent U.S. Pat. No. 4,261,943 describe such precautions.
For large output operations, however, the number of holes provided in a spinning nozzle must be very high. In this case, precautions for limiting surface adhesion of the freshly extruded filaments which pass through the air gap into a coagulation bath are completely insufficient.